Rotary machines such as steam and gas turbines used for power generation in mechanical drive applications are generally large machines consisting of multiple turbine stages. In turbines, high pressure fluid flowing through the turbine stages must pass through a series of stationary and rotating components and seals between the stationary and rotating components are used to control leakage. The efficiency of the turbine is directly dependent upon the ability of the seals to prevent is leakage, e.g., between the rotor and stator. Labyrinth-type seals are typically utilized in these designs. A labyrinth seal conventionally comprises a plurality of axially spaced seal teeth terminating in a tapered tip adjacent the rotary component, providing a tortuous path and, hence, an effective seal between high and low pressure regions on opposite axial sides of the seal. While labyrinth seals have proved to be quite reliable, their performance degrades over time as a result of transient events in which the stationary and rotating components interfere, rubbing the labyrinth teeth into a mushroom profile and opening the seal clearance.
Another type of seal used in many environments, including rotary machines, is a brush seal. Brush seals are generally less prone to leakage than labyrinth seals. The brush seal can also accommodate relative radial movement between fixed and rotational components, for example, between a rotor and a stator, because of the flexure of the seal bristles. Brush seals also generally conform better to surface non-uniformities. The result of using brush seals is better sustained rotary machine performance than is generally possible with labyrinth seals alone.
Combination labyrinth/brush seals have also been proposed. In these seals, a brush seal, i.e., a circumferentially extending array of elongated bristles, is disposed in combination with a labyrinth seal, either instead of one of the labyrinth teeth or in addition to the labyrinth teeth. The combination latyrinth/brush seal not only affords an effective seal but also provides a back-up seal should one of the different types of seals fails. It will be appreciated that while brush seals typically contact the rotary member, wear-in and rotary excursions sometimes open a clearance between the tips of the bristles and the rotary component, hence diminishing the effectiveness of the seal.